WO96/37921 and GB 2 328 086 have described systems in which coupled microstrip lines arc used to connect to a sensor wirelessly. In particular, they use two circular coupled transmission lines of nominally equal circumference so that intrinsically the amount of coupling will always vary with rotation. These systems have the drawback, however, that they are not very effective for microstrips significantly larger than ¼λ, where λ is the wavelength of the transmitted signal.
Another system is described in GB 2 368 470 which allows for 360° coupling between microstrip lines for diameters above ¼λ. These systems work by connecting together multiple sections of ¼λ coupled microstrips together and spreading these lines around a larger diameter. These systems suffer from the fact that the larger the diameter the more ¼λ elements that are needed and the feeding networks can then become quite cumbersome. They also suffer from the same problem as the aforementioned systems in that the amount of coupling will intrinsically vary with rotation.